By Brad Turman, PE
When the Aquila Dock coal handling facility needed to expand its rail lines, construction plans required an additional 15 to 20 feet (4.5 to 6 meters) of level land. The challenge? The site of the proposed line was located on a steep grade between an existing covered railway bridge and a live Norfolk Southern railway.
This location left Aquila Dock with very few options for creating a level space for the new line, as room for excavation and reinforcing any sort of wall was extremely limited. A 21-foot-high (6.4-meter-high) wall was required, but there was no room to bring in equipment for piling, and installing a grid-reinforced wall would have required undercutting the existing railway-causing the rail line to be shut down. Based on these factors, Aquila Dock officials decided that a precast, large block gravity wall would be the best choice for the project.
Searching for a solution, Aquila Dock turned to local Redi-Rock manufacturer Foster Supply, Inc. to learn about Redi-Rock's ability in this type of application. As a PE working for Foster Supply, I was able to provide free assistance to Aquila Dock in the preliminary design of the wall. The design criteria included creating a 4,500 square foot (418 square meter) gravity Redi-Rock retaining wall that would be constructed at the bottom of the slope and backfilled to create an additional 15 to 20 feet (4.5 to 6 meters) of level ground at the top of the wall. The wall would support an additional track approximately 16 feet (4.8 meters) back from the wall face. Train traffic on this section would be slow, about two to five miles per hour (3.2 to 5 kilometers per hour).
Redi-Rock walls are made of massive, precast concrete retaining wall blocks that weigh over one ton each. Due to the scale of the blocks, Redi-Rock can build tall gravity walls that often don't require geogrid and the extensive excavation that goes with it. Redi-Rock was the best solution for this project.
Through the use of various block types, I was able to recommend a design solution that met the height and geometry conditions required by this site. Additionally, for the loading condition, a 2500 per square foot (232 per square meter) strip load surcharge was included as part of the analysis.
The design included a reinforced concrete leveling pad with a concrete shear key at the toe to help resist sliding forces. The toe of the wall started 18 feet (5.5 meteres) below an overhang for the adjacent covered rail bridge. To achieve the necessary height, the wall design included 60 inch (1520 millimeter) base blocks, 60 inch (1520 millimeter) middle blocks, followed by 9 inch (230 millimeter) setback blocks and finished with 28 inch (710 millimeter) middle and top blocks. The wall was completely backfilled with imported ¾ inch (19 millimeter) limestone on a 1:1 influence beginning at the leveling pad and extending the full wall height.
Using the standard Redi-Rock wall system would have created approximately a 4 degree batter, which meant the top of the wall would have finished under the canopy overhang which was unacceptable. Instead, I recommended the Redi-rock 9 inch setback block to not only increase the design batter to approximately 26.6 degrees for this gravity structure, but also to ensure the wall cleared the canopy at the top. The 9 inch setback block enabled the project to meet our factor of safety with respect to overturning, and gave the owner the geometry they needed with the bottom and top of wall locations that had already been established.
Ultimately, Aquila Dock's own engineers were responsible for the final, approved wall design, which took into account suggestions made by Foster Supply. Additionally, Aquila Dock received approval from the railroad to proceed with the project.
Foster Supply acted as the construction manager for the entire project, providing all materials and installation, with the help of subcontractors J.R. Contractors. Installation began in 2008. Redi-Rock installed like giant Legos-each block interlocking with the one above through a system of knobs and grooves. The blocks were installed using a single piece of heavy machinery and a small crew. On this project, installation crews had to deal with very limited access to the site. They were only able to access the wall construction site from one location, so there was very little space to stage equipment or store material. Plus, excavation and installation had to take place while the adjacent railway was live. Material had to be stored away from the site and transported onsite in small amounts.
In total, the finished wall is approximately 4,500 square foot (418 square meter), with the tallest section standing 21 feet (6.4 meters) tall and stretching approximately 260 feet (50 meters) long. The completed wall used both Redi-Rock Cobblestone and Limestone textures.
Brad Turman is the owner of J.B. Turman Engineering in Lesage, West Virginia.
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